In order to understand the tensile fatigue characteristics of single carbon nanotube yarn (CNTY), experiments of fatigue loading and residual strength after different fatigue cycles were conducted. Results show that the tensile fatigue limit of the CNTY is similar to 68% of ultimate tensile strength (UTS). SEM figures show a typical fatigue process including crack initiation, crack propagation, and sudden fracture. Helix angles on the surface of CNTY decreased when the yarn underwent a certain number of tension-tension fatigue loading cycles, and the yarn was increasingly strengthened. Specifically, the strength and modulus of CNTY were increased by 21% and 468%, respectively, when the yarn was subjected to a 10(5) fatigue cycles at 68% UTS. The increase in residual specific strength after cyclic loading was found out to be the reason for the inflection point of the S-N curve. However, there were gaps between the surface layer and inner layer in the yarn. Cracks initiated along the gaps by shear force and friction during cyclic loading. Then, the failure of the inner CNT layers was caused by stress concentration at one of the relatively large cracks. A sudden fracture of the CNTY occurred eventually.